JOHANNESBURG (AP) — Petra Diamonds Ltd. says a diamond the size of a chicken egg has been found at South Africa's Cullinan mine.

The diamond may be among the world's top 20 high-quality gems. It was discovered Thursday at the mine northeast of Pretoria, South Africa.

Johan Dippenaar, the company's chief executive said in a statement Tuesday that the 507.55-carat gem was of "exceptional color and clarity.

No value has been give yet for the diamond, which weighs just over 100 grams.

The mine also produced the largest diamond ever found, the Cullinan, at 3,106 carats in the rough. That finished stone is set in Britain's Imperial Scepter as part of the Crown Jewels.